1. Field of the Invention
The instant invention relates to novel liquid membrane formulations, i.e., emulsions, which comprise an aqueous interior phase and a water-immiscible exterior phase; said water-immiscible exterior phase comprising an ethylene vinyl acetate copolymer and a solvent for this polymer. These compositions may additionally contain a water insoluble surfactant to stabilize the emulsions. In the most preferred embodiment, the aqueous interior phase comprises a strong acid, for example, from about 1 to 10 percent by weight sulfuric acid. These emulsions are useful in liquid membrane processes for the separation of dissolved components from aqueous solution. Emulsions of the instant invention are characterized as showing very low swelling when contacted with aqueous solutions, especially at higher temperatures and thus are especially effective for use in the treatment of sour water feed streams by the liquid membrane technique.
2. Description of the Prior Art
In U.S. Ser. No. 382,466, filed July 25, 1973, in the names of N. N. Li and R. P. Cahn, a process for removing the salt of a weak acid and a weak base from solution by means of the liquid membrane technology disclosed in U.S. Pat. Nos. 3,410,794, 3,617,546, and 3,779,907, all herein incorporated by reference, is disclosed. The process disclosed in U.S. Ser. No. 382,466 utilizes the liquid membrane technology to remove either the weak acid or weak base or their hydrolysis products from solution by permeating through the external phase of the liquid membrane emulsion and converting same into a nonpermeable form in the interior phase. Simultaneously the weak acid or weak base or the hydrolysis products thereof is stripped from solution by means of an inert gas. This process has been found to be most effective when run at high temperatures, for example 80.degree. C. It has been found, however, that, at temperatures in this range, many liquid membrane formulations, i.e., water-in-oil emulsions, are unsatisfactory, that is emulsion swelling, haziness of the treated water, etc. become problems. In the process of this invention, these problems are solved by means of novel formulations which may be used at temperatures of up to 95.degree. C.
U.S. Pat. No. 3,779,907 to Li et al demonstrates the use of Span 80 condensed with ethylene oxide as a preferred nonionic surfactant in liquid membrane technology. In the instant invention, an ethylene vinyl alkanoate is the copolymer employed ##STR1## wherein n may be from 1 to 20. Polymers of ethylene oxide and polymers of ethylene vinyl alkanoate cannot in any way be viewed as similar or analogous since clearly they are different in structure, composition and behavior due to the presence in the alkanoate of the classic ##STR2## group which is attached to the polyethylene backbone which is lacking in the polyoxide formed by polymerization of ethylene oxide which yields ethers, i.e., polymers with oxygen in the polymer backbone. Of greater importance, however, is the fact that Span 80 was not included in the examples of the instant specification since only the best systems were specifically demonstrated and during the course of developing the instant invention, it was discovered that Span 80 used alone is useless in the liquid membranes of the instant application when applied to high temperature removal of comtaminants. At high temperatures, hydrolysis of the ester linkages of Span 80 in the presence of acids and bases is a distinct possibility. If Span 80 is used alone and not in the presence of ethylene vinyl alkanoate, the emulsion so formed breaks down. However, when it is included in a composition comprising a solvent, ethylene vinyl alkanoate and Span 80, the emulsion is likely to function properly in spite of the fact that the Span 80 decomposes in the presence of acid or base at elevated temperatures.
If Span 80 is reacted with ethylene oxide, the resulting polymer is not in the class described in the instant invention as an ethylene vinyl alkanoate copolymer.
Span 80 is the condensation product [ester] of sorbitol and oleic acid. ##STR3## Obviously Span 80 is not an ethylene vinyl alkanoate copolymer.
If Span 80 is reacted with ethylene oxide, one gets: ##STR4## (it must be understood that only one isomer is represented above, reactions at the other --OH sites of Span 80 being equally possible and probable, the product being a combination of all the possible isomers), which is a compound commercially called Tween which also is not an ethylene vinyl alkanoate.